This invention relates to a tire having a tread of a rubber composition which contains a low molecular weight polyester plasticizer. Representative of various low molecular weight polyesters are, for example, polyester sebacate, triethylene glycol caprate-caprylate, triethylene glycol diheptanoate, triethylene glycol dipelargonate, triethylene glycol dipelargonate and triethylene glycol di-2-ethylhexoate.
High performance tires typically have rubber treads for which their surfaces intended to be ground-contacting are also intended to exhibit relatively high traction characteristics.
Accordingly, it is conventionally desired that the tread rubber composition of such high performance tire be relatively soft as evidenced by a relatively low hardness value, and/or to provide relatively high traction for the tread rubber as being predictive by a relatively higher hysteresis for the rubber composition as evidenced by higher tan delta and Jxe2x80x3 physical properties.
In the description of this invention, the term xe2x80x9cphrxe2x80x9d is used to designate parts by weight of a material per 100 parts by weight of elastomer. In the further description, the terms xe2x80x9crubberxe2x80x9d and xe2x80x9celastomerxe2x80x9d may be used interchangeably unless otherwise mentioned. The terms xe2x80x9cvulcanizedxe2x80x9d and xe2x80x9ccuredxe2x80x9d may be used interchangeably, as well as xe2x80x9cunvulcanizedxe2x80x9d or xe2x80x9cuncuredxe2x80x9d, unless otherwise indicated.
In accordance with this invention, a tire having a tread of a rubber composition comprised of, based upon 100 parts by weight of conjugated diene-based elastomer (phr),
(A) 100 phr of at least one diene-based elastomer, and
(B) about 1 to about 20, alternatively about 2 to about 15, phr of low molecular weight polyester selected from at least one of polyester sebacate having a molecular weight in a range of about 1000 to about 3000 so long as it has a melting point below 0xc2x0 C., triethylene glycol caprate-caprylate having molecular weight of about 430 formula weight, triethylene glycol diheptanoate having a molecular weight of about 388 formula weight, triethylene glycol dipelargonate having a molecular weight of about 420 formula weight and triethylene glycol di-2-ethylhexoate having a molecular weight of about 374 formula weight, preferably the polyester sebacate and the triethylene glycol caprate-caprylate.
Representative of said polyester sebacate is, for example, as PLASTHALL P-1070 from CP Hall. (melt point of about xe2x88x9222xc2x0 C.)
Representative of said triethylene glycol caprate-caprylate is, for example, PLASTHALL 4141 from C P Hall (melt point of about xe2x88x925xc2x0 C.).
Representative of said triethylene glycol diheptanoate is, for example, TegMeR 703 from C P Hall.
Representative of said triethylene glycol dipelargonate is, for example, TegMeR 903 from C P Hall.
Representative of said triethylene glycol di-2-ethylhexoate is, for example as TegMeR 803 from C P Hall Company.
The above molecular weights and indicated freeze (melt) points are values reported by the C P Hall Company.
A significant characteristic of the various triethylene glycol materials recited for use in this invention is that they have molecular weights being preferably below 750.
In practice, various conjugated diene-based elastomers may be used for the tire tread such as, for example, homopolymers and copolymers of monomers selected from isoprene and 1,3-butadiene and copolymers of at least one diene selected from isoprene and 1,3-butadiene and a vinyl aromatic compound selected from styrene and alphamethyl styrene, preferably styrene.
Representative of such conjugated diene-based elastomers are, for example, cis 1,4-polyisoprene (natural and synthetic), cis 1,4-polybutadiene, styrene/butadiene copolymers (aqueous emulsion polymerization prepared and organic solvent solution polymerization prepared), medium vinyl polybutadiene having a vinyl 1,2-content in a range of about 15 to about 90 percent, isoprene/butadiene copolymers, styrene/isoprene/butadiene terpolymers, styrene/isoprene copolymers and 3,4-polyisoprene.
A significant aspect of this invention appears to be, although the mechanism may not be entirely understood, that use of the low molecular weight polyester sebacate in a conjugated diene-based elastomer composition intended for use as a high performance tire tread has been observed to increase both a rubber composition""s 300 percent modulus and its hysteresis.
A significant aspect of this invention appears to be, although the mechanism may not be entirely understood, that use of the low molecular weight triethylene glycol caprate-caprylate in a conjugated diene-based elastomer composition intended for use as a high performance tire tread has been observed to reduce the room temperature hardness and RPA Gxe2x80x2 1% while maintaining the hysteretic properties and sometimes increasing the Strebler adhesion. As used herein the term xe2x80x9cRPAxe2x80x9d means rubber processing analyzer analytical equipment as produced by the Monsanto Company, and referred to as xe2x80x9cRPA 2000xe2x80x9d. The term xe2x80x9cRPA Gxe2x80x2 1 percentxe2x80x9d refers to the dynamic storage modulus xe2x80x9cGxe2x80x2xe2x80x9d at a one (1) percent strain (elongation) as determined by the RPA 2000 analytical equipment.
It is readily understood by those having skill in the art that the rubber composition would be compounded by methods generally known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials such as, for example, curing aids, such as sulfur, activators, retarders and accelerators, plasticizers additives, such as oils and resins, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants and reinforcing materials such as, for example, carbon black, silica and clay. As known to those skilled in the art, depending on the intended use of the sulfur vulcanizable and sulfur vulcanized material (rubbers), the additives mentioned above are selected and commonly used in conventional amounts.
Typical amounts of processing oils, if used, comprise about 1 to about 50 phr. Such processing oils can include, for example, aromatic, napthenic, and/or paraffinic processing oils. Typical amounts of antioxidants comprise about 0.5 to about 5 phr. Representative antioxidants may be, for example, diphenyl-p-phenylenediamine and others, such as, for example, those disclosed in The Vanderbilt Rubber Handbook (1978), Pages 344 through 346. Typical amounts of antiozonants comprise about 0 to 5 phr. Typical amounts of fatty acids, if used, which can include stearic acid comprise about 0.5 to about 3 phr. Typical amounts of zinc oxide comprise about 1 to about 10 phr. Typical amounts of waxes comprise about 0 to about 5 phr. Often microcrystalline waxes are used. The vulcanization is conducted in the presence of a sulfur vulcanizing agent. Examples of suitable sulfur vulcanizing agents include elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amine disulfide, polymeric polysulfide or sulfur olefin adducts. Preferably, the sulfur vulcanizing agent is elemental sulfur. As known to those skilled in the art, sulfur vulcanizing agents are used in an amount ranging from about 0.5 to about 4 phr, or even, in some circumstances, up to about 8 phr.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. Conventionally and preferably, a primary accelerator(s) is used in total amounts ranging from about 0.5 to about 4, preferably about 0.8 to about 1.5, phr. In another embodiment, combinations of a primary and a secondary accelerator might be used with the secondary accelerator being used in smaller amounts (of about 0.05 to about 3 phr) in order to activate and to improve the properties of the vulcanizate. Combinations of these accelerators might be expected to produce a synergistic effect on the final properties and are somewhat better than those produced by use of either accelerator alone. In addition, delayed action accelerators may be used which are not affected by normal processing temperatures but produce a satisfactory cure at ordinary vulcanization temperatures. Vulcanization retarders might also be used. Suitable types of accelerators that may be used in the present invention are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound.
The presence and relative amounts of the above additives are not considered to be an aspect of the present invention, unless otherwise indicated herein, which is more primarily directed to the utilization of low molecular weight polyesters in rubber compositions.
The mixing of the rubber composition can be accomplished by methods known to those having skill in the rubber mixing art. For example, the ingredients are typically mixed in at least two stages, namely, at least one non-productive stage followed by a productive mix stage. The final curatives are typically mixed in the final stage which is conventionally called the xe2x80x9cproductivexe2x80x9d mix stage in which the mixing typically occurs at a temperature, or ultimate temperature, lower than the mix temperature(s) than the preceding non-productive mix stage(s). The rubber and fillers such as carbon black and optional silica and coupler, and/or non-carbon black and non-silica fillers, are mixed in one or more non-productive mix stages. The terms xe2x80x9cnon-productivexe2x80x9d and xe2x80x9cproductivexe2x80x9d mix stages are well known to those having skill in the rubber mixing art.
The following examples are presented to illustrate the invention and are not intended to be limiting. The parts and percentages are by weight unless otherwise designated.